Traction control systems of motor vehicles limit wheel slipping in which one or more drive wheels overrun their traction surfaces. Slipping occurs when more torque is applied to a drive wheel than can be withstood by its traction surface for correspondingly moving the vehicle. The excess torque causes a sudden increase in drive wheel rotational speed with respect to its traction surface, referred to herein as wheel slipping.
Traction, which is measured as a force, is a function of wheel slip, which is measured as a percentage of overall drive wheel rotation in excess of rolling contact with the traction surface. A small percentage of wheel slip is needed to fully exploit the available traction force, but larger percentages of wheel slip reduce the traction force. Accordingly, wheel slipping, i.e., large percentages of wheel slip, actually reduces the amount of power that can be used to move a vehicle.
Traction control systems regulate drive power to the drive wheels by limiting the total drive power reaching the drive wheels or by influencing the distribution of drive power between the drive wheels. Power limiting systems regulate the delivery of power to a group of drive wheels, and power distributing systems divide the power in different proportions between the drive wheels.
For example, engine output power limiting systems regulate the delivery of drive power by controlling various engine functions including ignition, air intake, fuel intake, and exhaust. Engine controllers already regulate some or all of these functions, so little additional hardware is required to implement traction control. However, throttle controls are often preferred for directly overriding operator commands to the engine. Drive line power limiting systems interrupt the flow of power between the engine and the drive wheels by applying a braking force to the drive line or by temporarily disconnecting it.
Active or passive differentials can be used to influence the distribution of drive power between drive wheels. Active differentials have external controls that vary either frictional resistance to relative rotation between drive wheels (i.e., differentiation) or the speed ratio at which they are interconnected. Passive differentials develop frictional resistance to differentiation as a function of either the amount of differentiation or the amount of torque being transmitted.
Power distributing systems resist one drive wheel from slipping in advance of another by distributing more torque to the drive wheel having better traction but do not prevent both drive wheels from slipping together. Power limiting systems resist either or both drive wheels from slipping but do not exploit traction differences between the drive wheels.
Wheel braking systems have been used for both limiting the total drive power reaching the drive wheels and varying the distribution of the drive power between the drive wheels. However, the wheel brakes are not well suited for performing either function. The application of individual wheel brakes can produce shocks in the drive line or reflect excess torque between drive wheels resulting in drive line instabilities known as "hunting". Use of the wheel brakes for traction control accelerates their wear. Engine output power can often overwhelm the power-absorbing capacities of the wheel brakes. The application of the wheel brakes requires the generation of fluid pressure and its controlled conversion into mechanical braking torques, which can delay appropriate braking responses.
U.S. Pat. No. 5,269,390 to Glover et al. discloses a traction control system that combines an engine output power limiting system with a passive differential. Wheel slip is measured, and the engine output power limiting system reduces the drive power with respect to operator demand in response to a measure of wheel slip above a threshold. The passive differential is of the viscous coupling type that generates frictional torque opposing high rates of differentiation. The threshold is set to a much higher target value when only one drive wheel slips to allow the limited slip differential to operate properly.
However, viscous coupling type differentials only oppose high rates of wheel slip, and this limits the traction force available to the slipping drive wheel. The additional torque that can be delivered to the non-slipping wheel is also limited by activation of the engine output power limiting system at the higher target value of wheel slip. On the other hand, the higher target value delays any needed response of the engine output power limiting system to excessive wheel slip.